A lot of things change after you sell a book. One of the major ones is that you never get any long uninterrupted stretches of time when you can focus on just one project. I mean you might get a week or three, but an entire month is rare. And that's because everything keeps coming back to you! You finish a draft and send it off and then after a few months you get an edit letter. Or you do the major edits and then you have to look at copy edits.
And meanwhile, even as all of this is happening, you're going back and forth with your editor about your next book. And you've also got something with your agents. And you're also trying to work on your passion projects.
It's not time-consuming, not exactly, since you still only need to write for a few hours a day. But it's extremely distracting. You can't just sit down for three months and inhabit the world of a single book. Instead you always have to jump out and go back into another world--often one that you haven't touched for a few months (or even a year). It's definitely different, and I still haven't quite wrapped my head around it yet.
Of course, I am a little different in that I have been known to write first drafts very fast. For instance the middle grade novel I've been revising (cross your fingers for this one, since I think it'll go on sub sometime in the next six months or so) was something that I wrote in four or five days.
That though, just means that everything piles up in the revision phase (which still takes me a decent amount of time. Anyway, we will see. At least things are more exciting 🙂